The following example program should give you a good start:
http://zone.ni.com/devzone/explprog.nsf/6c163603265406328625682a006ed37d/97383d950b2b38fe862568940057d3b4?OpenDocument
For your application you will want to pay particular attention to the Excel_ChartChartWizard function call.
Also, if you want to write your ActiveX client from scratch in a new project, Use the ActiveX Automation Wizard (from the tools menu) to create an instrument for communicating with the server application.
Amaury R.
Applications Engineer
National Instruments